For example, in an internal combustion engine, much waste heat is generated when driving. On the other hand, by applying heat when starting, start-up becomes smooth. Therefore in some cases a heat storage unit is provided so that the waste heat when driving is stored and is then used for warming up when starting.
As a device related to a conventional heat storage unit, there is for example, the one disclosed in Japanese Examined Patent Application, Second Publication No. Hei 5-4244. This heat storage unit has an inner case, a heat insulator for covering the outside of this inner case, and an outer case for covering the outside of this heat insulator. It also has a box provided with an inlet for introducing a fluid on one side of a central part, and provided with an outlet for discharging a fluid on the other side of the central part, a heat reservoir with heat storage material enclosed in an envelope and which is arranged in the box, and a core around which this heat reservoir is wrapped. The heat reservoir is wrapped around the core in a spiral shape with a plurality of spacers intervening, to thereby form crevices which become fluid passages for flowing the heat exchange fluid.
In the abovementioned heat storage unit, the heat storage material is enclosed in the envelope to form the heat reservoir, and the crevices which become fluid passages are formed with the spacers intervening, and this heat storage material is wrapped around the core in a spiral shape. As a result, there are problems in that, there are many parts, and since manufacture is complex, manufacturing cost is increased.
Moreover, another problem is that a core is always required for forming the crevices which become the fluid passages with spacers intervening, and for wrapping the heat reservoir around this in a spiral shape, and by the amount of this core, the heat storage capacity is decreased.
Therefore, it is an object of the present invention to provide a heat storage unit wherein the number of parts can be reduced, and manufacture is simplified so that manufacturing cost can be reduced, and also the heat storage capacity can be adequately maintained, and high performance can be attained, and a manufacturing method therefor.
On the other hand, in conventional heat storage units including the one mentioned above, as a heat storage material, for example, a PCM (Phase Change Material) is used. By using the latent heat of fusion when the heat storage material is converted from liquid form into solid form, a large amount of heat energy can be stored with a small size.
However, in the heat storage material including the abovementioned PCM, in some cases the density differs between liquid form and solid form and the volume varies corresponding to the heat storage condition. In a sealed space therefore, the volume variation occurring between the solid phase and liquid phase may cause deformation of the components of the heat storage unit. As a solution, generally, a specified quantity of air is enclosed in the heat storage material filling space for filling with the heat storage material, so that the volume variation is absorbed by this air. However, in this way, in order to absorb the volume variation of the heat storage material by air without causing any deformation of components, the quantity of air to be enclosed must be set quite large, and much more than the volume variation of the heat storage material. Consequently, this may cause other problems such as a decrease in heat capacity, a decrease in heat transfer coefficient, and oxidation and deterioration of the heat storage material by the oxygen in the air.
Moreover, regarding the heat storage unit, in the heat storage process, when the heat from the fluid has been stored sufficiently to give the same temperature as that of the fluid, heat is not efficiently stored thereafter, resulting in wasteful passage resistance. Therefore, it is necessary to detect the condition of the heat storage material from the outside and to switch the fluid passage to a passage for bypassing the heat storage unit. Accordingly, there is a problem in that parts such as a sensor, three way valve, control unit and actuator are necessary, so that the number of parts is increased and cost is high.